In the prior art, exemplified by Carlson patent, U.S. Pat. No. 2,820,841 (1958), solar cells employing cadmium sulphide are disclosed. These cells were fabricated by vacuum deposition of CdS on Nesa glass, in the form of a thin microcrystalline layer, and by depositing over the CdS layer a further layer of a material comprising monovalent cations of a metal selected from group 1B of the periodic table. The latter is preferably cuprous sulphide, Cu.sub.2 S. In accordance with our prior application for U.S. patent, Ser. No. 303,365, filed Nov. 3, 1972, CdS, Cu.sub.2 S photo-voltaic or solar cells are produced by spraying suitable solutions, in atomized form, on conventional Nesa glass. Our process requires far less time than does vacuum deposition, but in addition produces cells which are superior in certain respects.
If solar cells are to be utilized for large scale production of power, areas of such cells are required in terms of square miles, and the cost of fabricating large areas of cells must be comparable with the cost of producing power by conventional systems, in terms of cost of providing a conventional system capable of producing the same power. On this basis, the economic feasibility of a large scale solar cell system depends in considerable part on the efficiency of the cells, i.e., the ratio of electrical output power to solar energy input, and this in turn is a function of the resistivity in ohms per square of the Nesa glass, which forms a negative electrode for the solar cells, and of its transmissivity to solar energy. One problem then is to provide low resistivity layers on glass, far lower than is the case for ordinary Nesa glass or than is known in the prior art, but which possesses high transmittivity to most components of solar radiant energy. We have produced such films on glass, as SnO.sub.x, with resistivities of about 10 ohms per square. If the CdS layers have rather large specific resistivities, as for example in the range 10.sup.3 to 10.sup.5 ohm-cm, and if the SnO.sub.x layer and the CdS layer are transparent and clear, and not cloudy, cell efficiencies of 5 percent are then attained, and values as high as 8 percent are feasible. Output voltages of about 400. M.V. are regularly achieved. Radiation transmission through the SnO.sub.x is 92 percent in the spectral range of interest. Through the glass SnO.sub.x combination it is 78 percent.
The problem of producing high efficiency devices is not solely one of producing low resistance per square SnO.sub.x coatings. The latter can be accomplished by employing thick coatings of SnO.sub.x, but in that case tranmissivity to light is impaired. There is involved a tradeoff, in terms of resistance per square, and transmissivity to desired radiation frequencies, and discrimination against infra-red rays.
A further problem relates to production of output electrodes. If copper is applied over the Cu.sub.2 S layer, the low resistance of the copper renders the high resistance of the Cu.sub.2 S of no operational significance. We have found that introducing oxygen into the Cu.sub.2 S layer (hereinafter described), is beneficial. To accomplish this, the Cu.sub.2 S layer can be sprayed to a required thickness, and then the spraying continued with the addition of oxygen in the form of CuSO.sub.4 to form a layer superposed on the Cu.sub.2 S layer. The latter acts to protect the Cu.sub.2 S layer from atmospheric contamination, and may itself be protected by a further superposed layer of copper.
Proceeding as above described, it is estimated that power outputs of 128,000 K.W. (peak) at 5 percent efficiency per square mile of cells can be achieved.
The problem remains of producing the required square footage of cells, at reasonable cost and in a reasonable time. In accordance with the present invention, float glass manufacturing plants, which are capable of large scale production of glass sheet in a continuous process, are to be modified to include spraying or other types of deposition of the glass sheet, as it is made, with the requisite coatings, i.e., SnO.sub.x, CdS and Cu.sub.2 S. The finished coated glass might then be cut into panels, perhaps 4.times.8 feet in size, which would be provided with electrodes, shipped to an installation site, and there mounted and interconnected to a power delivery and storage system.
There is a major advantage in so proceeding in that the glass sheet, as initially fabricated, is at a higher temperature then it is at any step of our process, and the steps of the process can be achieved at successively lower temperatures. It follows that our process can be achieved with minimum addition of heat during the coating process, as the latter proceeds. It is necessary to maintain the float tanks and furnaces at the temperatures requisite for the process, but it is not necessary to add heat to the glass itself, and in fact the molten material has heat added to it by the glass. This represents a large saving in energy, in comparison with processes in which cold glass panels are heated and then coated. It also represents a large saving in time to complete the process over that involved in commencing the process with cold panels of glass, and heating the glass to that required for coating with SnO.sub.x as the first step in the process of making large scale solar cells.
The approximate cost of a thermal electricity generating plant is at least $250 per kw. It is estimated that a solar cell installation produced by the method described herein and generating equivalent average power would cost approximately the same. However, the cost of energy storage is not included in the estimate.
Certain of the degrading factors which operate in the case of cells of the Carlson type have been found absent in the present cell. For example, infra red energy appears to degrade the Carlson type, i.e., vacuum deposited cells. Our sprayed cells, having low resistivity SnO.sub.x coatings, are exposed to radiation via the glass surface, and our low resistivity SnO.sub.x is found to discriminate against the infra red energy, i.e., for wavelengths longer than 1.5 .mu.m. transmission falls off rapidly, but the coating is highly effective in passing frequencies above the infra red.